Urinary incontinence, or involuntary leakage of urine, is a condition which affects a significant portion of the population, but is especially prevalent in women (Melville et al. 2005). Urinary incontinence may present in different forms, such as stress incontinence and urge incontinence, or a mixture of forms. Continence relies on a number of factors, including relaxation of the detrusor muscles around the bladder wall, and proper activity of the urethral sphincter muscles and structures (including blood vessels) around the urethra (Smith et al. 2006). Abnormal muscle activity in the urinary bladder or urethral sphincter may also cause other problems, for instance overactive bladder or incomplete bladder emptying.
Abnormality of the detrusor muscles of the bladder, resulting in abnormal bladder function such as overactive bladder or urinary incontinence, may be caused by impairments, for instance, to the detrusor muscle activity or to the neurological connections to the detrusor muscle (Semins and Chancellor 2004). Monitoring of detrusor muscle activity provides useful information for a urologist in diagnosing or monitoring bladder function in patients with conditions such as urinary incontinence or overactive bladder. Conventional urodynamics procedures do not provide direct measurements of bladder muscle activity. Other imaging modalities have been utilized for this purpose, but also fail to provide detailed information on the activity of the detrusor muscle. For instance, ultrasonography has been used to determine bladder volume during bladder filling and voiding experiments in spinal cord injured animals (Keirstead et al. 2005). In another study, multi-slice echo-planar imaging was used to assess bladder volume and morphology.
Deficiency or abnormality of the urethral sphincter, resulting in urinary incontinence, can be caused by a number of different factors, for instance loss of urethral compression and support after pelvic surgery, childbirth, and pelvic trauma; lumbosacral neuropathy; and loss of muscle strength due to aging (Macura et al. 2006). Evaluation of the urinary sphincter muscles and other structures, such as blood vessels, may be achieved using a variety of known techniques, for instance urodynamics, cystourethroscopy, cystourethrography, ultrasonography and magnetic resonance imaging (Macura et al. 2006). There remains a need for improved devices and methods which allow accurate monitoring and/or imaging of the activity and status of urethral sphincter muscles and structures.
Urinary incontinence may often be associated with poor pelvic floor muscle strength and/or poor urinary sphincter muscle strength, which may or may not be a consequence of vaginal delivery during pregnancy. It is recognized that strengthening the pelvic floor muscles and/or urinary sphincter muscles may be beneficial, either during pregnancy to aid in delivery and prevent subsequent urinary incontinence issues, or in non-pregnancy related situations, for instance to improve or reduce urinary incontinence (Vasconcelos et al. 2006; de Oliveira et al. 2007). Using biofeedback during pelvic floor muscle or urinary sphincter muscle strengthening exercises is one method to improve the outcome of such exercises. Several methodologies are described in the literature to provide biofeedback monitoring during these kinds of exercises, which may include electromyography (EMG), perineometry, ultrasound, or measurement of intravaginal pressure (Peschers et al. 2001).
Near-infrared spectroscopy (NIRS) is a technique that has found use in a number of different biomedical applications, for instance monitoring of blood oxygenation and hemoglobin content, assessment of cerebral activity and evaluation of different tissues. In the near-infrared spectrum (particularly between 700 to 1100 nm), the primary absorbers of light in the context of the body are by chromophores in hemoglobin, oxyhemoglobin, water and lipids. In practice, NIR light penetrates tissues such as skin, bone, muscle and soft tissue where it is absorbed by the chromophores. These chromophores vary in their absorbance of NIRS light, depending on changes in oxygenation. Light in the visible spectrum (ie. 450-700 nm) penetrates tissue only short distances because it is usually attenuated by different tissue components. In the near-infrared spectrum, tissue penetration is much higher, up to several centimeters, allowing non-invasive monitoring of different tissue properties. For example, US Patent Publication 2006/0276712 discloses a method and devices for monitoring bladder detrusor muscle using near-infrared light through the skin.
The unique relation between the transparency of tissue to near infrared light and the specific absorption spectra of individual chromophores forms the basis of clinical near infrared spectroscopy. The principal chromophore of interest in studies using NIRS is hemoglobin which has a different extinction coefficient (absorption characteristic) across the NIR spectrum when oxygenated (O2Hb) and deoxygenated (HHb). Cytochrome-c-oxidase (CCO), the terminal enzyme of the mitochondrial respiratory chain, also absorbs light differently across the NIR spectrum depending on its redox status although the contribution of CCO to overall absorption is considerably less (approximately one tenth) than that of hemoglobin.
The majority of NIRS instruments used clinically are continuous wave units with lasers that transmit pulses of multiple wavelengths of light into the tissues, and sensors to detect the photons returning that are not absorbed. The changes in absorption at discrete wavelengths generate raw optical data that can be converted by software algorithms into concentration changes for each chromophore using a modification of the Lambert-Beer law. The related algorithms and software necessary for NIRS data to be used clinically also accommodate a number of limitations posed by the nature of human tissue, including the pathlength of NIR light and loss of photons undetected because of scattering beyond the field of view.
The full extent of the field through which light scatters is generally unknown in vivo, so that the initial concentration of each chromophore is generally unknown. Hence, clinical NIRS generally measures absolute changes in concentration relative to the initial baseline concentration. With real time sampling and graphic conversion of data, patterns of change in chromophore concentration and magnitudes of change are derived which can be used to infer physiologic change occurring within the tissue interrogated. Such changes include: an increase or decrease in O2Hb (an indirect measure of oxygen content); an increase or decrease in the total hemoglobin (change in blood volume); an abrupt decrease in O2Hb with simultaneous increase in HHb (ischemia); and a gradual decrease in O2Hb and increase in HHb (hypoxia). As cytochrome-c-oxidase drives>95% of O2 consumption and the synthesis of adenosine triphosphate (ATP) within mitochondria, changes in CCO redox status provide information relating to electron transport and oxidative phosphorylation at a cellular level. Interpretation of NIRS data that includes changes in O2Hb, HHb and CCO signals can offer important insights into oxygen utilization, energy dynamics and cellular well being.
Continuous wave NIRS instruments typically incorporate the following: a) at least one pulsed laser diode for each chromophore being sampled. Typically the lasers emit light in 1, 2 or 4 wavelengths in the 729 to 920 nm near infrared wavelength range with a 5 nm spectral width and pulse duration of 100 nanoseconds at 2 kHz cycle frequency; b) Fiberoptic bundles that transmit light from the source to a tissue interface (probe or patch) and back to the instrument; c) Optodes in the tissue interface that emit light into the tissue and receive the photons returning; d) Photon counting hardware (photomultiplier or photodiode); d) Computer with software containing algorithms for converting raw optical data into chromophore concentrations, storing and displaying data; e) A visual display where NIRS data are typically displayed graphically against time. Some instruments provide a choice from multiple wavelengths, and the option to use more than one data channel to allow comparison of different sites is available; a few incorporate additional spatial resolution that allows measurement of the ratio of oxygenated to total tissue hemoglobin which can be displayed as a measure of tissue oxygenation; and monitoring in the form of a regional map using arrays of emitters and receivers is possible.